Conventionally, an image-forming method using a so-called photosensitive toner functioning as both a developer and a photoconductor is known. In such a known method, a thin layer comprised of the toner having a photoconductivity is formed on a conductive drum, and the toner thin layer is then exposed to light to obtain a toner image.
FIG. 2 shows an example of the prior art image-forming method. Referring to FIG. 2, a toner thin layer 22 of a photosensitive toner 23 charged to have a given polarity by triboelectric charging or the like is formed on a toner retaining member 21 having a conductivity (the photosensitive toner 23 may be charged to have a given polarity by corona charging or the like after forming the toner thin layer). Then, exposure 24 according to an original image is applied to the toner thin layer 22. Accordingly, the toner is made conductive by the exposure 24, and the charge possessed by the toner is dissipated through the conductive toner retaining member 21 which is grounded, or a charge having a reverse polarity is injected into the toner. Thus, an electrostatic latent image 25 is formed on the toner thin layer. On the other hand, a transfer material 26 is brought into contact with the toner thin layer on which the latent image 25 has been formed, and a bias voltage is applied from a back side of the transfer material 26 as pressing a transfer roller 27 against the transfer material 26. As a result, the transfer material 26 is charged to have a polarity reversed to or the same as that of the toner not exposed, thereby transferring the toner onto the transfer material 26. In the former case where the transfer material is charged to have the reversed polarity, a positive image is formed, while in the latter case where the transfer material is charged to have the same polarity, a negative image is formed.
However, the above-mentioned transfer step is different from a general transfer process for transferring a toner image only formed on a photosensitive body in a xerography system which is represented by a Carlson process. That is, the transfer step as shown in FIG. 2 includes separating of the toner having a given polarity only from the toner thin layer having opposite polarities. If such a transferring and separating operation is carried out in the same manner as the above-mentioned xerography system wherein the toner image only formed on the photosensitive body is transferred, that is, if the transfer material is brought into contact with the toner thin layer on the conductive drum, and the transfer roller to which a bias voltage is applied from the back side of the transfer material is pressed against the transfer material to transfer the toner image onto the transfer material at the pressing point, and just thereafter separate the transfer material from the toner thin layer owing to the stiffness and the deadweight of the transfer material, the toner corresponding to a non-image portion is unnecessarily transferred onto the transfer material together with the toner corresponding to an image portion, resulting in the generation of fog. Thus, it is difficult to steadily form a sharp toner image having a high density with no fog.